No search was conducted on the subject matter of this specification in the U.S. Patent and Trademark Office or in any other search facility. I am unaware of any prior art more relevant to the subject matter of this specification than that which will be set forth below.
Many modern day automotive engine control systems employ oxygen gas sensors to detect the level of oxygen in the exhaust gases leaving the internal combustion engine after a combustion process therein. The oxygen sensors measure the oxygen concentration in a manner which indicates whether the exhaust gases have oxygen contained therein as oxygen or whether the gases are oxygen deficient. This indicates whether the combustion mixture being fed to the engine is rich in fuel, the exhaust gases will be oxygen deficient, or lean in fuel, the exhaust gases will have oxygen therein. By knowing whether the engine is operating rich or lean, the device metering fuel into the air going to the engine can be adjusted by electronic controls so that a proper amount of fuel is being fed to the internal combustion process.
In order to improve the accuracy of the exhaust gas sensors, it has been proposed that a heater element be used in conjunction with the sensing element so that the sensing element is always at a particular reference temperature and thus give consistent readings over long periods of time. The so-called "self-heated" oxygen sensor configuration has advantages over the unheated version by having a very short light-off time (that is, it begins to operate properly within a relatively short period of time) and greater immunity to performance instability caused by exhaust gas deposits and temperature fluctuations.
The present specification is directed to circuits which use only three wires to effect a self-heated sensor construction. The circuits disclosed herein can incorporate not only a heater, a sensing element, but also a stabilizing thermistor, and still require only three external leads.